Design techniques for single mode planar microwave filters, such as broadside edge coupled filters, have long been known. Implementation of such planar microwave filters is often achieved using microstrip and stripline fabrication techniques. For example, Zhuang et al., "Microstrip Disk Cavities Filter Using Gap Capacitance Coupling", IEEE MTT-S Digest, pp. 551-554 (1988) discloses a circular, single mode bandpass filter which is implemented in microstrip. Referring to FIG. 1a of the instant application, there is illustrated the single mode filter of Zhuang et al. The device comprises a linear array of circular resonant cavities 10 with a single set of conduction leads 12. Energy is coupled into and out of the resonators 10 along the axis defined by the resonators 10 and conduction leads 12.
Single mode planar filters such as those disclosed by Zhuang et al. are of limited utility for most high performance microwave applications due to their typically high insertion losses and the impracticality of designing single mode filters with passbands of less than 5%. For example, communication satellite frequency multiplexers typically require the use of dual mode cavity or dielectric resonator filters to realize self equalized, quasi-elliptic responses. These filters have passbands that are often less than 1% but have the disadvantages of large size and high cost. In addition, they are not compatible with superconductor implementation. Filters of this type are discussed in U.S. Pat. No. 4,453,146.
Planar ring resonators capable of supporting dual resonance modes are disclosed in Wolff, "Microstrip Bandpass Filter Using Degenerate Modes of a Microstrip Ring Resonator", Electronics Letters, Vol. 8 No. 12, pp. 143-144 (1972). However, Wolff's filter does not allow orthogonal modes to be coupled into and out of the ring resonator independently. Rather, a perturbation is used to generate the second mode from the single mode which is input to the device.
Planar rectangular filters capable of supporting dual orthogonal modes are known. Referring to FIG. 1b of the instant application, there is illustrated the device disclosed in U.S. Pat. No. 3,796,970. The device is based on a rectangular planar filter 14 having sides which are dimensioned to support two orthogonal resonant modes. Two pairs of conductive leads 16,18 couple energy into and out of these two orthogonal modes. However, coupling between the resonant modes is achieved only by means of an additional conductive lead 19 which is external to the resonator. Since modes are not coupled internal to the filter, quasi-elliptic and self equalized functionality are precluded.
Soviet Union patent 1,062,809A discloses a planar rectangular resonator having two sets of capacitively coupled input/output conductive leads. No means to support or control internal coupling of the resonant modes is provided.
Japanese patent 58-99002 discloses an adjustable notch in a single mode slotline ring resonator for tuning the center frequency and bandwidth of a microwave filter. Signals are coupled into and out of the resonator by means of conductor leads which are located in a plane separate from that of the resonator ring.
Guglielmi, "Microstrip Ring-Resonator Dual-Mode Filters", distributed at the Workshop on Microwave Filters for Space Applications by ESA (European Space Agency)/ESTEC in June, 1991, discloses a dual-mode filter cell having two transmission poles and two transmission zeros.
The instant inventors published portions of the instant invention in "Miniature Dual Mode Microstrip Filters", IEEE MTT-S International Microwave Synposium Digest, Vol. 2, pp. 443-446 (June 1991).